It is often difficult to remove the remaining water when preparing products, such as sugars, sugar alcohols and similar sensitive as well as hygroscopic products in dry form, such as in powder form.
One of the reasons is the high content of compounds with hydrophilic groups in most of the above materials, rendering the products hygroscopic.
Another reason is the tendency of some of said materials to form supersaturated solutions, wherefrom it is difficult to precipitate and isolate solid products.
Supersaturated and other concentrated solutions are very often highly viscous. Consequently they are difficult to handle and have the tendency to stick to the apparatus.
All these properties further impede the removal of water, e.g. by evaporation or drying, since problems arise during the heating of the material. An equal distribution of heat is, for example, not ensured, thus risking local overheating. During the heating the material can be destroyed or deteriorate e.g. by burning, carmamelization, denaturation or another form of decomposition.
Several methods for drying products in order to remove the remaining water are known, such as spray drying, drum drying, freeze drying or flash drying.
For spray drying, the solution to be dried is fed into a chamber in the form of tiny drops. The falling drops are dried by means of hot air so that the drops are transformed into a dry powder before they reach the bottom of the chamber. Spray drying cannot be used if the solution has the tendency to remain liquid, either as an supersaturated solution or in form of a melt, during the drying process, where the temperature is usually about 60.degree. C., since the material accumulates on the walls of the spray drying chamber.
Drum drying is normally performed at temperatures about 100.degree. C. At this temperature many materials occur in form of a melt and thus they cannot be transformed into a solid product. During drum drying the product accumulates on the warm surfaces, causing overheating with subsequent destruction or deterioration of the material.
With conventional flash drying water is removed almost instantaneously from wet, solid particles, said particles being dispersed at high speed in a warm stream of gas. In flash drying the temperature of the drying air is above 100.degree. C., rendering this drying method unsuitable for drying heat-sensitive products.
It is evident that neither spray drying, drum drying nor flash drying are suitable for the preparation of solid, dry products, such as some sugars and some sugar alcohols, other heat-sensitive products and/or those difficult to crystallize.
The only known methods suitable for drying such materials are freeze drying and microwave vacuum drying. These methods are, however, expensive, since their operational costs are high, especially with regard to energy consumption and capital costs.
The German Offenlegungsschrift No. 34 07 374 discloses a method for preparing dried products from sucrose syrup. According to this method the pre-concentrated sucrose syrup with a dry matter content of at least 70% is heated for a short period, such as below 60 sec, to a very high temperature, and the warm material is expanded to a concentrated syrup with a dry matter content of at least 90%. This syrup is transformed into a dry, pourable product by sudden cooling and subsequent release of the remaining water during crystallization.
This method is limited to easily crystallizable materials with a positive enthalpy of crystallization, i.e. materials crystallizing during cooling. The method is consequently unsuitable for amorphous products and other materials difficult to crystallize. Furthermore the material is subjected to high temperatures of about 135.degree.-155.degree. C., thus rendering the method unsuitable for heat-sensitive materials.
U.S. Pat. No. 1.250.496 discloses a process for drying grain and other coarse, granular materials, where the grain is heated and then subjected to first a low vacuum and thereafter a high vacuum. The process involves expensive equipment and the process cannot be used for drying particle-free, syrup-like materials.
U.S. Pat. No. 3.206.866 discloses a method and apparatus for dehydrating food employing geothermal steam. The food is fed into a vacuum chamber through an air lock and is transported through said chamber by means of several conveyor belts and is finally removed from the vacuum chamber through an air lock. The food is heated inside the vacuum chamber. The latter step renders this method unsuitable for dehydration of syrup-like materials since the material is locally overheated resulting in its deterioration.
U.S. Pat. No. 4.574.495 discloses a drying apparatus with a vacuum chamber, wherein the material to be dried is transported by means of conveyor belts. This apparatus is also equipped with means for supplying heat to the inside of the vacuum chamber. Therefore this apparatus also deteriorates a syrup-like material due to overheating.
Finally EP Patent Application No. 0.231.584 Al discloses a drying apparatus including a screw conveyor. The apparatus is heated by means of a heating mantle. Therefore also this apparatus is unsuitable for drying heat-sensitive materials, such as syrup-like material.
Other patents, such as SE-PS No. 342.896, SE-PS NO. 374.811, U.S. Pat. No. 3.698.098 and U.S. Pat. No. 1.161.603, disclose various methods and apparatuses for drying materials, such as wood panels, protinaceous materials, heat-sensitive parachutes of synthetic fibres and colloid substances, respectively.
GB-PS No. 1.498.119 discloses a process for drying and expanding a past, which is extruded into a vacuum chamber. The temperature of the paste is between 60 and 125.degree. C. This process is, however, unsuitable for drying a substantially particle-free, syrup-like material. The known process is difficult to control, if the extruder aperture has a diameter of less than 0.25 mm. In order to effect an adequate control of the process the aperture should be of a diameter of from 0.5 to 5 mm.
DE-OS No. 2.444.099 discloses a method for preparing dextrose powder in dry form containing a major amount of .beta.-dextrose anhydride. This method, however, involves a grafted crystallization and is not usable in case on non-crystallizing materials, such as a mixture of oligosaccharides prepared according to the concurrent DK patent application No. 1592/88, filed Mar. 23, 1988.
P. E. Anderson and J. Risum (Introduktion til Levnedsmiddel-Teknologien, vol. 1, 3. edition, p. 333, 1982, Polyteknisk Forlag, Copenhagen) disclose a conventional flash evaporator. The evaporator is, however, only suitable for the preparation of concentrated, still liquid products.